1. Field of Use
The present invention relates diagnostic apparatus and more specifically, apparatus for enabling diagnosis of a failed data processing unit.
2. Prior Art
A number of data processing system include apparatus which enables a "bad" unit or device to be deconfigured or logically disconnected from the system. When this occurs, the rest of the system is unable to communicate with the unit over any of the interfaces to which it connects. Hence, it is not possible to access any of the pertinent status registers for determining the reason for failure.
One way of gaining access to the bad unit is to include another path for accessing registers. However, this has been found to be costly and to require that a certain minimum amount of control logic circuits within the unit be operative to enable the transfer of signals across the interface.
Another disadvantage of the above is that in accessing internal information from the "bad" unit, it is possible to alter the state of the unit. This is particularly true when the "bad" unit is a processor.
Accordingly, it is a primary object of the present invention to provide apparatus for enabling access to internal information stored in a unit which has been logically disconnected from a system.
It is a more specific object of the present invention to provide an arrangement which provides internal information from a processing unit which has been logically disconnected from a system and inhibited from performing further operations.